Hexagonal crystal structure piezoelectric materials such as AlN and ZnO are of commercial interest due to their piezoelectric and electroacoustic properties. Beneficial properties in this regard include a high quality factor, moderate coupling coefficient, moderate piezoelectric constant, high acoustic velocity, and low propagation losses. In addition to these characteristics, AlN thin films are chemically stable and compatible with various integrated circuit fabrication technologies, thereby making AlN an attractive material for fabrication of electroacoustic devices, including bulk acoustic wave (BAW) devices such as bandpass filters and the like.
A primary use of electroacoustic technology has been in the telecommunication field (e.g., for oscillators, filters, delay lines, etc.). More recently, there has been a growing interest in using electroacoustic devices in high frequency sensing applications due to the potential for high sensitivity, resolution, and reliability. However, it is not trivial to apply electroacoustic technology in certain sensor applications—particularly sensors operating in liquid/viscous media (e.g., chemical and biochemical sensors)—since longitudinal and surface waves exhibit considerable acoustic leakage into such media, thereby resulting in reduced resolution.
In the case of a piezoelectric crystal resonator, an acoustic wave may embody either a bulk acoustic wave (BAW) propagating through the interior (or ‘bulk’) of a piezoelectric material, or a surface acoustic wave (SAW) propagating on the surface of the piezoelectric material. SAW devices involve transduction of acoustic waves (commonly including two-dimensional Rayleigh waves) utilizing interdigital transducers along the surface of a piezoelectric material, with the waves being confined to a penetration depth of about one wavelength. BAW devices typically involve transduction of an acoustic wave using electrodes arranged on opposing top and bottom surfaces of a piezoelectric material. In a BAW device, different vibration modes can propagate in the bulk material, including a longitudinal mode and two differently polarized shear modes, wherein the longitudinal and shear bulk modes propagate at different velocities. The longitudinal mode is characterized by compression and elongation in the direction of the propagation, whereas the shear modes consist of motion perpendicular to the direction of propagation with no local change of volume. The propagation characteristics of these bulk modes depend on the material properties and propagation direction respective to the crystal axis orientations. Since shear waves exhibit a very low penetration depth into a liquid, a device with pure or predominant shear modes can operate in liquids without significant radiation losses (in contrast with longitudinal waves, which can be radiated in liquid and exhibit significant propagation losses). Restated, shear mode vibrations are beneficial for operation of acoustic wave devices with fluids because shear waves do not impart significant energy into fluids.
Certain piezoelectric thin films are capable of exciting both longitudinal and shear mode resonance. To excite a wave including a shear mode using a standard sandwiched electrode configuration device, a polarization axis in a piezoelectric thin film must generally be non-perpendicular to (e.g., tilted relative to) the film plane. Hexagonal crystal structure piezoelectric materials such as (but not limited to) aluminum nitride (AlN) and zinc oxide (ZnO) tend to develop their polarization axis (i.e., c-axis) perpendicular to the film plane, since the (0001) plane typically has the lowest surface density and is thermodynamically preferred. Certain high temperature (e.g., vapor phase epitaxy) processes may be used to grow tilted c-axis films, but providing full compatibility with microelectronic structures such as metal electrodes or traces requires a low temperature deposition process (e.g., typically below about 300° C.).
Low temperature deposition methods such as reactive radio frequency magnetron sputtering have been used for preparing tilted AlN films having angles that vary significantly with position over the area of a substrate. FIG. 1 is a simplified schematic of an axial sputtering deposition apparatus arranged to eject metal atoms from a target 2 (adjacent to a cathode (not shown)) toward a substrate 4 supported by a substrate holder 6 that is substantially parallel to the target 2 in a reactive gas-containing environment. The target 2 and the substrate 4 are aligned with one another and share a single central axis 8; however, a typical geometry of sputtering deposition results in a cosine distribution 10 of piezoelectric material molecules (e.g., AlN molecules created by metal atoms reacting with nitrogen in the sputtering gas) being received by the substrate 4. This phenomenon leads to a c-axis direction of the deposited piezoelectric material that varies with radial position, from an angle of zero (corresponding to a vertical c-axis) at the center of the substrate 4, to a c-axis direction with a tilt angle that increases with distance from the center.
The above-described variation with radial position of c-axis direction of a deposited piezoelectric material is disclosed by Stan, G. E., et al., “Tilt c Axis Crystallite Growth of Aluminium Nitride Films by Reactive RF-Magnetron Sputtering,” Digest Journal of Nanomaterials and Biostructures, vol. 7, no. 1, pp. 41-50 (2012) (hereinafter, “Stan”). FIG. 2A is a schematic representation (reproduced from Stan) of a rocking curve measurement geometry of an AlN film structure obtained by radio frequency magnetron sputtering in a reactive gas environment in an axially aligned planar sputtering system without tilting a 50 mm Si substrate. FIG. 2A shows that the AlN film structure according to Stan exhibits zero c-axis tilt angle at the center, and a radially symmetrical variation of tilt angle of crystallites in the AlN film structure, analogous to a circular “race track” with banked walls. FIG. 2B is a plot of tilt angle versus distance from center (also derived from Stan) for the AlN film structure described in connection with FIG. 2A, showing a nearly linear variation of tilt angle with increasing distance away from a center of the AlN film structure, to a maximum tilt angle of about 6.5 degrees near the margins of the 50 mm wafer. One effect of the lack of uniformity of c-axis tilt angle of the AlN film structure over the substrate is that if the AlN film-covered substrate were to be diced into individual chips, the individual chips would exhibit significant variation in c-axis tilt angle and concomitant variation in acoustic wave propagation characteristics. Such variation in c-axis tilt angle would render it difficult to efficiently produce large numbers of resonator chips with consistent and repeatable performance. Moreover, use of a target surface axially aligned with a substrate holder that is parallel to the target surface enables attainment of only a limited range of c-axis tilt angles, as evidenced by the 0-6.5 degree tilt angle range shown in FIG. 2A.
Before describing other techniques for preparing tilted AlN films, desirable regimes for c-axis tilt angle (or angle of inclination) will be discussed. An electromechanical coupling coefficient is a numerical value that represents the efficiency of piezoelectric materials in converting electrical energy into acoustic energy for a given acoustic mode. Changing the c-axis angle of inclination for hexagonal crystal structure piezoelectric materials causes variation in shear and longitudinal coupling coefficients, as shown in FIG. 3. FIG. 3 embodies plots of shear coupling coefficient (Ks) and longitudinal coupling coefficient (Kl) each as a function of c-axis angle of inclination for AlN. It can be seen that a maximum electromechanical coupling coefficient of shear mode resonance in AlN is obtained at a c-axis angle of inclination of about 35 degrees, that a pure shear response (with zero longitudinal coupling) is obtained at a c-axis angle of inclination of about 46 degrees, and that the shear coupling coefficient exceeds the longitudinal coupling coefficient for c-axis angle of inclination values in a range of from about 19 degrees to about 63 degrees. The longitudinal coupling coefficient is also zero at a c-axis angle of inclination of 90 degrees, but it is impractical to grow AlN at very steep c-axis inclination angles. For electroacoustic resonators intended to operate in liquids or other viscous media, it would be desirable to provide piezoelectric films with a c-axis tilt angle sufficient to provide a shear coupling coefficient that exceeds a longitudinal coupling coefficient—in certain embodiments, at a c-axis tilt angle in which the longitudinal coupling coefficient approaches zero, or at a c-axis-tilt angle at or near a value where shear coupling is maximized. Thus, for an electroacoustic resonator including an AlN piezoelectric layer, it would be desirable to provide a c-axis tilt angle in a range of from about 19 degrees to about 63 degrees, and particularly desirable to provide a c-axis tilt angle of 35 or 46 degrees.
Various low temperature deposition methods that have been devised for growing AlN films at c-axis tilt angles greater than those attainable with the axial sputtering apparatus of FIG. 1 are described in connection with FIGS. 4A-4C. FIG. 4A (which is adapted from Moreira, Milena De Albuquerque, “Synthesis of Thin Piezoelectric AlN Films in View of Sensors and Telecom Applications,” 2014, Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 1651-6214; 1160) (hereinafter, “Moreira”) is a simplified schematic of an off-axis sputtering deposition apparatus arranged to eject metal atoms from a target surface 12 toward a substrate 14 supported by a substrate holder 16 that is substantially parallel to the target surface 12, with central axes of the substrate 14 and the target surface 12 being parallel but offset relative to one another, with an angle θ representing an angle between a central axis 18 of the target surface 12 and a center of the substrate 14. A distribution 20 of piezoelectric material molecules created by reaction of metal atoms and gas is received by the substrate 14, resulting in a tilted c-axis direction of deposited piezoelectric material (including tilted ‘columns’ of piezoelectric material with a preferential growth direction due to alignment with the tilted flux), with the c-axis tilt angle of the piezoelectric material varying with respect to position along the substrate 14. In particular, a portion of the deposited piezoelectric material that is closer to the central axis 18 of the target surface 12 will exhibit a c-axis tilt angle that is shallower than a portion of the piezoelectric material that is farther from the central axis 18 of the target surface 12. Due to the lateral offset of the substrate 14 relative to the central axis 18 of the target surface 12, the off-axis sputtering deposition apparatus of FIG. 4A is capable of attaining piezoelectric films with c-axis tilt angles that are larger than those attainable with the apparatus of FIG. 1.
Additional low temperature deposition sputtering apparatuses capable of growing piezoelectric films with even larger c-axis tilt angles are described in connection with FIGS. 4B and 4C (which are also adapted from Moreira). FIG. 4B is a simplified schematic of a sputtering deposition apparatus arranged to eject metal atoms from a target surface 22 toward a substrate 24 supported by a substrate holder 26 that is non-parallel to the target surface 22 (i.e., wherein the substrate holder 26 is tilted by an angle θ relative to a plane parallel with the target surface 22), wherein a central axis 28 of the target surface 22 extends through a center point of the substrate 24. A distribution 30 of piezoelectric material molecules created by reaction of metal atoms and gas is received by the substrate 24, resulting in a tilted c-axis direction of deposited piezoelectric material (including tilted ‘columns’ of piezoelectric material with a preferential growth direction due to alignment with the tilted flux), with the c-axis tilt angle of the piezoelectric material varying with respect to position along the substrate 24. FIG. 4C is a simplified schematic of a sputtering deposition apparatus arranged to eject metal atoms from a target surface 32 toward a substrate 34 supported by a substrate holder 36 that is non-parallel to the target surface 32. A central axis 38B of the substrate 34 extends through a center point of the target surface 32, with a central axis 38A of the target surface 32 being separated from the central axis 38B of the substrate 34 by a first angle θ1, and with substrate holder 36 being tilted by a second angle θ2 relative to a plane parallel with the target surface 32. A distribution 40 of piezoelectric material molecules created by reaction of metal atoms and gas is received by the substrate 34, resulting in a tilted c-axis direction of deposited piezoelectric material (including tilted ‘columns’ of piezoelectric material with a preferential growth direction due to alignment with the tilted flux), with the c-axis tilt angle of the piezoelectric material varying with respect to position along the substrate 34.
Yet another method for growing a tilted c-axis AlN film involves two-step sputtering deposition as described by Moreira, including first step growth of an initial, non-textured seed layer at a relatively high process pressure while keeping the substrate at room temperature, followed by second step growth of a film at a lower process pressure and an elevated substrate temperature. FIG. 5A is a cross-sectional schematic view of a seed layer 44 exhibiting multiple textures deposited via the first sputtering step over a substrate 42, and FIG. 5B is a similar view of the seed layer 44 and substrate 42 of FIG. 5A following deposition via a second sputtering step of a tilted axis AlN film 46 over the seed layer 44. As described by Moreira, the seed layer exhibits different textures, most notably (103) and (002). Additionally, the film growth tends to follow the crystallographic texture of the seed layer, and the low pressure deposition in combination with a small distance between the target and substrate yields a directional deposition flux that results in competitive column growth in which cones having a c-axis along the direction of the deposition flux grow fastest. This results in a film with a c-axis lying in the plane of the deposition flux at any given point along the substrate. As noted by Moreira, even though there is no intentional tilt of the flux, the magnetron disposition at the target surface generates a “race track”, which in turn provides the tilted flux direction towards the substrate. Such a “race track” described by Moreira is understood to correspond to a radially symmetric variation of tilt angle of crystallites in the film structure, similar to that described hereinabove in connection with FIGS. 2A and 2B.
Each of the foregoing apparatuses and tilted piezoelectric material growth methods are understood to produce film-covered substrates exhibiting significant variation in c-axis tilt angle with respect to position on the substrate. As noted previously, one effect of a lack of uniformity of c-axis tilt angle of a piezoelectric film arranged on the substrate is that if the film-covered substrate were to be diced into individual chips, then the individual chips would exhibit significant variation in c-axis tilt angle and concomitant variation in acoustic wave propagation characteristics. Such variation in c-axis tilt angle would render it difficult to efficiently produce large numbers of resonator chips with consistent and repeatable performance.
Accordingly, there is a need for systems and methods for producing inclined c-axis hexagonal crystalline material films over large area substrates without significant variation in c-axis tilt angle, such as to enable economical production of bulk acoustic wave resonator structures with repeatable performance characteristics. It would further be desirable for such systems and methods to be compatible with integrated circuit fabrication technologies and enable fabrication of piezoelectric films with c-axis tilt angles sufficient to provide high shear coupling coefficients, so as to enable production of resonator devices suitable for use in liquids and other viscous media.